The Danube running through Vienna is safe to swim in: https://www.wien.gv.at/forschung/laboratorien/umweltmedizin/wasserhygiene/badewasserqualitaet/index.html h

The Danube running through Vienna is safe to swim in: https://www.wien.gv.at/forschung/laboratorien/umweltmedizin/wasserhygiene/badewasserqualitaet/index.html https://www.wien.gv.at/forschung/laboratorien/umweltmedizin/... The water quality doesn't protect you from the current or the heavy boat traffic through. So most swimming is done in a disconnected side channel called "Alte Donau".